The objective of this project is to define the roles played by thyroid hormones and prolactin in the etiology and biology of breast cancer. Studies are being made of nuclear triodothyronine, cell membrane prolactin, and cytoplasmic estrogen receptors, and of mitochondrial alpha-glycerophosphate dehydrogenase in experimental rat mammary carcinomas induced by N-nitrosomethylurea, and human breast cancer tissues. The influence of thyroid function and circulating prolactin levels on the receptor proteins and the thyroid hormone-inducible mitochondrial enzyme will be studied. Clinical investigations will be concerned with the influence of previous drug administration on tumor receptor levels, particular attention being paid to those known to alter plasma prolactin and estrogen levels.